Large storage tanks are used to store hydrocarbon-based fuels for future use. In many cases, tanks are arranged close to one another, forming tank farms. Probes are installed in or onto the tank to monitor both the amount of liquid in the tank and the condition of the liquid itself.
Hydrostatic tank gauging ("HTG") systems have been sold commercially since the early 1980's. HTG standards are described in the "Manual of Petroleum Measurement Standards," (American Petroleum Institute, November 1994), specifically, "Chapter 16--Measurement of Hydrocarbon Fluids by Weight or Mass," incorporated herein by reference for all purposes.
The typical HTG system includes 3 pressure sensors installed along the tank wall. The first sensor is placed near the bottom of the tank. The second sensor is placed about 10 feet above the bottom of the tank. The third sensor is attached to the tank roof.
The first sensor measures hydrostatic pressure of the liquid column and is used for calculating the mass of liquid in the tank. The difference in pressure readings between the first sensor and the second sensor is proportional to the average density of the liquid stored in the tank, thus providing an indication of the level of liquid within the tank. The roof pressure sensor is used for measuring the vapor pressure. The vapor pressure measurement taken by the third sensor is used for compensation of the vapor pressure influence. The typical HTG system also includes a temperature sensor that is installed roof without loss of seal or reference point. The probe consists of a series of pipes that are connected together, end to end, by large flanges. These flanges are welded to the pipes and then are connected to the mating flange of the subsequent pipe, with six or more bolts. The flanges may leak if the bolts connecting the flanges are not torqued evenly or torqued properly. Improper torquing has been a problem in the past and has required additional checking procedures by qualified inspectors. Manufacturing of the flanges also necessitates additional pressure testing, to assure the welding quality critical for a submersible probe construction. Finally, the dozens of nuts and bolts needed to make the necessary connections means that the installation procedure is time consuming and costly.
The MACStick probe also has a special thick flange located between some of the sections. The special thick flange is used as the basis for pressure and temperature sensor installation. The MACStick probe includes a cubical compartment head that is bolted onto the tank roof. The head contains an analog transmitter. The transmitter conveys pressure and temperature signals in 4 to 20 mA current signals. A MACStick analog transmitter can handle two pressure and two temperature sensors. Thus the standard MACStick probe configuration, containing 4 pressure and 4 temperature sensors, must include two analog transmitters in the probe head. In addition, each MACStick probe must have a remote central processing unit ("RCPU") attached to it. The RCPU provides analog to digital conversion of the current transmitters' signals, data processing and calculations.
While useful, the MACStick probe has several disadvantages beyond those mentioned previously. First, most liquid tanks are installed with stilling wells. Stilling wells, typically, are pipes that are fitted within the tank along the centerline. Stilling wells are used for a variety of purposes including the structural integrity of the tank itself. Some stilling wells are perforated to allow liquid from the tank to enter the well. The remaining stilling wells are not perforated. The non-perforated stilling wells are used in halfway between the first pressure sensor and the second pressure sensor. The temperature sensor is used to calculate the standard density and standard volume of the liquid, converted back to 60 F. (or 15 C.).
There are disadvantages with the typical HTG system. These disadvantages include inconvenient and costly installation of the pressure sensors along the tank wall. This installation requires either hot tapping or, in the alternative, taking the tank out of service until the sensors are installed. Further, additional valves must be installed between the tank and the sensor to allow for periodic calibration and maintenance of the pressure sensors.
Another disadvantage of the typical HTG system is that tank bulging affects the measurements. Tank bulging occurs when the walls of the tank deflect under the weight of the liquid contained therein. There is no practical possibility for precise measurement of the distance between the pressure sensors' actual location on the deformed tank wall. Consequently, there is an inherent error in the density calculation, and a corresponding error in the liquid level measurement. This error increases with the age and deterioration of the tank. Furthermore, because the density is measured in the relatively thin layer of liquid, there is more uncertainty in the result, which is calculated as hydrostatic pressure divided by density. Weather factors, such as wind, heating of the sensor by the sun, and temperature differences between environment (sensor body) and liquid (sensor diaphragm), may significantly affect the measured result. Finally, the zero drift of the pressure sensors requires periodic manual calibration and, if not calibrated often, the zero drift is a potential source of error.
Yet another prior art HTG system is sold by Measurement and Control Systems ("MACS"), located at Ackerstein Bldg., 103 Medinat Hayehudim Street, Herzliya Pituach 46766, Israel. The device is called MACStick, and it consists of a probe that is inserted into the middle of the tank. The probe itself runs from the bottom through the roof of the tank where the probe is fitted through a collar on the roof to allow for movement of the jurisdictions having environmental laws that forbid the venting of hydrocarbon vapors into the atmosphere. The MACStick probe, having large flanges positioned at regular intervals, cannot be used in tanks without stilling wells because the seals on the floating roofs cannot operate around the MACStick flanges. However, if the stilling well is not perforated, placing the MACStick probe within the stilling well does no good because the stilling well prevents the liquid from reaching the probe. The stilling well does not provide an adequate sample to the probe of the contents of the tank because the sample from the stilling well is not representative of the liquid dispersed throughout the tank. Consequently, the MACStick probe can only be used on liquid tanks having perforated stilling wells.
In addition to the limited applicability of the MACStick device, there are still other disadvantages. For example, pressure sensors are located within some of the flanges. The pressure sensor location on the flange that has a relatively large diameter means that a rather long channel is needed to provide the surrounding liquid access to the sensor diaphragm. These long channels are easily plugged or clogged by viscous or impure liquids stored in the tanks.
The last significant problem with the MACStick device is its reliance on analog transmitters with limited capacity, particularly when more than four pressure and temperature sensors are required for an application. Finally, MACStick, having analog transmitters, has a much lower tolerance to electromagnetic interference ("EMI") of the analog signal transmission. This design always requires more cables and also requires separate digital processing units for each sensor.
There is, therefore, a need in the art for a probe that is both easy to manufacture, easy to install, and that has the ability to function with floating roofs without violating environmental laws. It is an object of the present invention to provide an easy to make, easy to install, and easy to use probe that will work with any storage tank.